Charged particle beam apparatus

ABSTRACT

It is an object of the present invention to obtain an image which is focused on all portions of a sample and to provide a charged particle beam apparatus capable of obtaining a two-dimensional image which has no blurred part over an entire sample. In order to achieve the above object, the present invention comprises means for changing a focus condition of a charged particle beam emitted from a charged particle source, a charged particle detector for detecting charged particles irradiated from a surface portion of said sample in response to the emitted charged particle beam, and means for composing a two-dimensional image of the surface portion of the sample based on signals on which said charged particle beam is focused, said signals being among signals output from the charged particle detector.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.11/515,732, filed Sep. 6 2006 now U.S. Pat. No. 7,329,868, which is aContinuation of U.S. application Ser. No. 11/108,731, filed Apr. 19,2005, now U.S. Pat. No. 7,109,485, which is a Continuation of U.S.application Ser. No. 10/681,116, filed Oct. 9, 2003, now U.S. Pat. No.6,936,818, which is a Continuation of U.S. application Ser. No.10/356,498, filed Feb. 3, 2003, now U.S. Pat. No. 6,653,633, which is aDivisional of U.S. application Ser. No. 09/612,302, filed Jul. 7, 2000,now U.S. Pat. No. 6,538,249, claiming priority of Japanese ApplicationNos. 11-195375, filed Jul. 9, 1999; 11-296246, filed Oct. 19, 1999, and2000-170407, filed Jun. 2, 2000, the entire contents of each of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a charged particle beam apparatus, andmore particularly to a charged particle beam apparatus having a functionof properly adjusting the focus of an image obtained by charged particlebeam irradiation.

A charged particle beam apparatus such as a scanning electron microscopeis suitable for measuring or observing patterns formed on asemiconductor wafer, which has been becoming finer. Incidentally,samples used for such a purpose have taken a shape extending morethree-dimensionally as semiconductor wafers have been multilayered. Forexample, currently, deeper contact holes have been formed in a sample.

SUMMARY OF THE INVENTION

A charged particle beam apparatus such as a scanning electron microscopethinly converges a beam and irradiates it onto a sample, which requiresproper focusing of the beam on the sample. As semiconductor wafers havebeen multilayered, however, the distance between the surface of a sampleand the bottom surface of a contact hole therein, for example, hasbecome longer, causing a problem that the surface of the sample and thebottom surface of the contact hole have different focal distances. Thatis, focusing a beam onto the surface of the sample causes the bottomsurface of the contact hole to be out of focus, producing a blurredimage of the bottom surface of the contact hole.

Incidentally, Japanese Laid-Open Patent Publication No. 5-128989 (1993)discloses a technique which irradiates an electron beam onto athree-dimensional object while changing the focus of the beam, andextracts the contours of in-focus portions of the object to construct athree-dimensional image. In such an apparatus, however, when atwo-dimensional image of a sample including the bottom of a contact holeis observed, for example, it is not possible to observe the details ofthe sample surface and the bottom portions of the contact hole sinceonly the contour of the contact hole is indicated.

Japanese Laid-Open Patent Publication No. 5-299048 (1993) disclosesanother example which basically extracts contours of an object havingconcave/convex portions and produces a pseudo three-dimensional image ina representation similar to a contour chart.

The technique disclosed in Japanese Laid-Open Patent Publication No.5-299048 (1993) performs differential processing on an image obtained bychanging a focus, and extracts portions whose differential values exceeda preset extraction level.

This process is repeated on a plurality of images obtained by changing afocus, and finally the extracted portions are combined to extractcontours of concave/convex portions of the imaged object. At that time,no consideration is given to portions whose differential values are lessthan the extraction level. Furthermore, since the extraction level,which is an evaluation level for determining a contour, depends on theS/N ratio of an image and the shape of the object, it is not possible toset a constant value for all portions. When there are two types ofconcave/convex portions in an image as shown in FIG. 16, for example,since a shape 1601 has a steep inclination, its in-focus portion has alarge differential value, while since a shape 1602 has a moderateinclination, its in-focus portion has a small differential value.Therefore, if the same extraction level is applied to both shapes, theshape 1602 may not be extracted, depending on a selected extractionlevel. Thus, failing to set an appropriate extraction level produces anunextracted contour portion. Although the example in FIG. 16 shows onlytwo types of concave/convex portions, an actual image has an infinitenumber of concave/convex portions. It is impossible to set an extractionlevel by which all of these contour portions are extracted. Since theabove example extracts contour portions of each image separately, and noconsideration is given to relationships between images whose portionshave been extracted, when the extracted portions are combined to producea composite image without setting an appropriate extraction level, someportions in the composite image may be left indefinite, or portionsextracted from two or more images may overlap, as shown in FIG. 17. Thatis, in the invention disclosed in Japanese Laid-Open Patent PublicationNo. 5-299048 (1993), it is very difficult to set an extraction level,and in addition, no consideration is given to a method for processingextracted portions between images.

It is an object of the present invention to obtain an image which isfocused on all portions of a sample or a certain two-dimensional area ofa sample and to provide a charged particle beam apparatus capable ofobtaining a two-dimensional image which has no blurred part over anentire sample.

In order to achieve the above object, a charged particle beam apparatusin accordance with the present invention comprises a charged particlesource, a scan deflector for scanning a charged particle beam emittedfrom the charged particle source on a sample, means for changing a focusof the charged particle beam emitted from said charged particle source,a charged particle detector for detecting charged particles obtained ata portion of said sample irradiated with the charged particle beam, andmeans for composing a two-dimensional image of the sample as viewed froma direction of said charged particle beam source, based on signals onwhich said charged particle beam is focused, said signals being amongsignals output from the charged particle detector.

With this configuration, it is possible to select charged particlesemitted from a two-dimensional area of a portion in focus from amongcharged particles obtained from an entire sample, and use the chargedparticles to form a sample image. That is, since a sample image can beconstructed based on charged particles focused on an entire area or aspecific two-dimensional area-in a beam scan area, it is possible tocompose a two-dimensional image that is focused on the charged particlebeam scan area or a specific two-dimensional area thereof.

Another mode according to the present invention utilizes differentialvalues or changes in a Sobel value at same coordinates of a plurality ofimages obtained by changing a focus, and uses a pixel value of theoriginal image of an image which has a maximum value of those values tocompose an image. This eliminates setting of unstable parameters as wellas overlapping of portions extracted from the same image or more thanone image for composition, resulting in composition of a full-focusedimage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a configuration of a scanning electronmicroscope;

FIG. 2 is a graph showing changes in a focus evaluation value aselectron lens conditions are changed;

FIG. 3 is a diagram for describing focus deviations, which are a problemto be solved by the present invention;

FIG. 4 is schematic diagram for describing creation of a composite imageaccording to the present invention;

FIG. 5 is a flowchart showing a flow of processes for extractingin-focus portions and creating a composite image according to thepresent invention;

FIG. 6 is a flowchart showing another flow of processes for extractingin-focus portions and creating a composite image according to thepresent invention;

FIG. 7 is a flowchart showing still another flow of processes forextracting in-focus portions and creating a composite image according tothe present invention;

FIG. 8 is a flowchart showing a flow of processes in which imageacquisition, extraction of in-focus portions, and creation of acomposite image are performed in parallel according to an embodiment ofthe present invention;

FIG. 9 is a diagram showing indication examples for displaying compositeimages on a real time basis according to the present invention;

FIG. 10 is a diagram showing an example of measuring a length using acomposite image according to the present invention;

FIG. 11 is a schematic diagram showing a composing process according tothe present invention;

FIG. 12 is a schematic diagram showing a method for calculating a heightdifference between two given points in a composite image according tothe present invention;

FIG. 13 is a graph showing a relationship between an excitation currentand a focal distance;

FIG. 14 is a diagram showing an indication example for a display devicefor an apparatus according to an embodiment of the present invention;

FIG. 15 is a diagram showing an example of a GUI screen for an apparatusaccording to an embodiment of the present invention;

FIG. 16 is a diagram showing a method for detecting a concave/convexcontour;

FIG. 17 is a diagram showing composition results in concave/convexcontour detection;

FIG. 18 is a schematic diagram showing an image composing process bydetermining an in-focus degree using a signal of type different from oneused for composition detected at the same time;

FIG. 19 is an example of a composite image obtained by characteristicquantity comparison by use of a plurality of different types of signals;

FIG. 20 is a schematic diagram of a configuration of a scanning electronmicroscope having a plurality of detectors;

FIG. 21 is a diagram showing the principle of a scanning electronmicroscope suitable for composing an image using sample images obtainedfor each focus by changing a focus in a stepwise manner;

FIGS. 22(A) to 22(D) are diagrams for describing a line profile for eachsample image obtained by changing a focus;

FIG. 23 is a diagram showing the concept of full-focused imagecomposition;

FIG. 24 is a diagram showing an embodiment of full-focused imagecomposition;

FIG. 25 is a diagram showing another embodiment of full-focused imagecomposition;

FIG. 26 is a diagram showing a configuration of a focus determinationmeans;

FIG. 27 is a diagram showing a configuration of a noise determinationmeans;

FIG. 28 is a diagram showing a configuration of a composing means;

FIG. 29 is a flowchart of full-focused image composition;

FIG. 30 is a diagram showing an embodiment of full-focused imagecomposition;

FIG. 31 is a diagram showing a configuration of a preprocessing means210 for full-focused image composition.

FIG. 32 is a diagram showing another configuration of a preprocessingmeans 210 for full-focused image composition;

FIG. 33 is a diagram showing an embodiment of full-focused imagecomposition;

FIG. 34 is a diagram showing another embodiment of full-focused imagecomposition;

FIG. 35 is a diagram showing still another embodiment of full-focusedimage composition;

FIG. 36 is a graph showing relationships between an observationmagnification and a focal depth when the same acceleration voltage anddifferent beam resolutions are applied;

FIG. 37 is a graph showing relationships between an observationmagnification and a focal depth when the same beam resolution anddifferent acceleration voltages are applied;

FIG. 38 is a diagram showing a schematic configuration of a scanningelectron microscope used to describe an embodiment of the presentinvention;

FIG. 39 is a diagram showing a schematic configuration of anotherscanning electron microscope used to describe an embodiment of thepresent invention;

FIG. 40 is a flowchart showing a control flow for acquiring a series ofimages each having a different focus with the number of images to beacquired specified;

FIG. 41 is a flowchart showing a control flow for acquiring a series ofimages each having a different focus with a focal depth specified; and

FIG. 42 is a flowchart showing a control flow for acquiring a series ofimages each having a different focus with a focal range specified.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although embodiments according to the present invention will bedescribed below with reference to a scanning electron microscope, whichis a charged particle beam apparatus, the invention is not limited tothis specific charged particle beam apparatus but can be applied toother charged particle beam apparatuses such as an FIB (Focused IonBeam) apparatus which scans an ion beam on a sample to obtain a sampleimage.

FIG. 1 is a diagram showing a scanning electron microscope to which thepresent invention is applied. This scanning electron microscopeincorporates an automatic focus control function. In FIG. 1, referencenumerals 101 and 102 denote a sample stage and a sample to be imaged onthe sample stage, respectively; 104 denotes a cathode; 105 represents ascanning coil; 106 represents an electron lens; 108 denotes a scanningcoil control circuit; and 109 denotes a lens control circuit.

An electron beam 114 is scanned on the sample 102 by the scanning coil105, and electrons emitted from the sample 102 are detected by adetector 103. A signal S1 from the detector 103 is input to an ADconverter 107, which converts the signal into a digital signal S2.

The digital signal S2 is fed to an image processing processor 110 whichperforms image processing such as differential processing of an imageand extraction of characteristic quantities, and sends the results to acontrol computer 111.

The processed image is also sent to a display device 112 where the imageis displayed. A focus control signal S3 from the control computer 111 isinput to the lens control circuit 109 so that the lens control circuitcan adjust the exciting current for the lens 106 to perform focuscontrol.

Numeral 113 denotes an input means connected to the control computer111. This scanning electron microscope, as configured above, performsautomatic focus control by automatically setting focal conditions of theelectron lens to optimum values. Specifically, the scanning electronmicroscope scans a plurality of frames while changing the electron lensconditions and calculates and evaluates focus evaluation values usingthe detection signals of secondary electrons and reflected electrons soas to set optimum values to the electron lens conditions. FIG. 2 showschanges in a focus evaluation value as an electron lens condition isvaried.

Here, differential values between pixels or the like are used as focusevaluation values. Specifically, the sum of differential values iscalculated for each frame captured while changing the lens condition,and a lens condition in which the sum is maximized is regarded as anin-focus condition. In FIG. 2, since the focus evaluation value is atmaximum (Fmax) when the exciting current for the electron lens is f, thecondition f is determined as an in-focus condition.

A scanning electron microscope having an automatic focus controlfunction such as one described above has the following problems:

First, since a focus evaluation value is calculated by applying acertain set process to a frame image or detection signals, an acquiredvalue is applied to a frame image as a whole, that is, no considerationis given to local in-focus conditions. More specifically, when a samplehas concave/convex portions, in-focus conditions are different betweenthe upper surface and the bottom surface. However, the automatic focuscontrol determines an optimum condition for either the upper surface orbottom surface as an optimum value for both, or it calculates a mediumcondition as an optimum value.

Next, the automatic focus control takes time. As seen from FIG. 2, sincea plurality of frame images must be read to find an optimum focusevaluation value (that is, to find the maximum value in FIG. 2), ittakes a few or a few tens of seconds to complete the control operation.

Embodiments according to the present invention provide a preferredscanning electron microscope capable of solving especially the above twoproblems. The configurations of apparatuses embodied according to thepresent invention will be described in detail below.

First Embodiment

FIG. 3 is a diagram used to describe focus deviations, which are aproblem to be solved by the present invention. when a semiconductorsample with a contact hole therein is scanned by a scanning electronmicroscope, if the electron beam is focused on the surface of thesemiconductor sample, the bottom surface of a contact hole having a highaspect ratio becomes out of focus. However, if the electron beam isfocused onto the bottom surface of the contact hole, on the other hand,the surface of the sample becomes out of focus. Automatic focus controlfunctions currently incorporated in scanning electron microscopes cannothandle a local focus deviation such as this one, and can only calculatea sample surface or some average position as a focal position.

FIG. 4 is a schematic diagram for describing creation of a compositeimage according to the present invention. Using the semiconductor samplewith contact holes therein described in FIG. 3, two images are captured:one in which a focal position is set an the surface of the semiconductorsample and the other in which a focal position is set on the bottomsurface of a contact hole. Then, in-focus portions can be extracted fromeach image so as to produce a composite image, which is atwo-dimensional image focusing on all surfaces of the sample. These twoimages are registered in, for example, two frame memories.

FIG. 5 is a flowchart showing a flow of processes for extractingin-focus portions and creating a composite image according to thepresent invention. Two images Ai,j and Bi,j whose focal positions areshifted from each other are captured. The focal positions are shifted bysending the focus control signal S3 from the control computer 111 to thelens control circuit 109 to adjust exciting current for the electronlens 106, as described using FIG. 1. Step 501 creates a differentialabsolute value image for each of the two captured images (ΔxyAi,j forAi,j and ΔxyBi,j for Bi,j). A differential absolute value image iscreated using values acquired by adding the absolute value of thedifference between a pixel and another pixel shifted n pixels from thepixel in the x-direction to the absolute value of the difference betweenthe same pixel and another pixel shifted n pixels from the pixel in they-direction, as indicated by the formula (1).Δxy A i, j=Δx A i, j+Δy A i, jΔxy B i, j=Δx B i, j+Δy B i, jΔx A i, j=|A i, j−A i+n, j|, Δx B i, j=|B i, j−B i+n, j|Δy A i, j=|A i, j−A i, j+n|, Δy B i, j=|B i, j−B i, j+n|  (1)

Before using differential absolute value images created at step 501 asin-focus evaluation references, they are smoothed at step 502 tosuppress noise influence. Step 503 determines which image is in focus,and creates a composite image. Here, the in-focus evaluation isperformed based on the formula (2). That is, step 503 compares pixelvalues at same coordinates in the two differential absolute value imagessmoothed at step 502, and determines that a pixel of an original imagewhich has a larger corresponding pixel value is in focus.[ΔA]i, j≧[ΔB]i, j→C i, j=A i, j[ΔA]i, j<[ΔB]i, j→C i, j=B i, j  (2)

A composite image Ci,j is composed of in-focus portions of Ai,j andBi,j. FIG. 5 illustrates composition using two images, and compositionusing n images can be performed by sequentially repeating the sameprocess on a series of image pairs.

FIG. 11 is a schematic diagram showing a composing process according tothe present invention. The figure illustrates an example in which pixelvalues from a Sobel filter are set as in-focus evaluation references.Like image differential, the Sobel filter is used to extract edgeinformation of an image, and when a pixel value processed by a Sobelfilter is large, this means that changes in pixel values around thepixel are large. That is, the pixel is in focus and is hardly blurred.Numeral 1101 indicates a plurality of images captured by changing afocus, and 1102 indicates images obtained by processing each image 1101by use of a Sobel filter. Each of the images 1101 is registered in oneof a plurality of prepared frame memories.

Pixels Sg1 through Sg5 at same coordinates in the plurality of images1102 registered in the frame memories are compared, and of those pixels,a pixel of the largest value is extracted. Supposing that the pixel ofthe largest value is Sg2, a pixel value g2 of the original image of thepixel Sg2 is projected to a pixel at same coordinates in the compositeimage. A composite image 1103 is acquired by repeating this process forall coordinates of the image to select pixels of largest values, andarranging them to form a two-dimensional image.

FIG. 6 is a flowchart showing another flow of processes for extractingin-focus portions and creating a composite image according to thepresent invention. Steps 601 and 602 create differential absolute valueimages and smooth the images, respectively, as at steps 501 and 502 inFIG. 5. Next, step 603 determines which one of the two images is infocus, and creates a composite image. Here, the in-focus evaluation isperformed based on the formula (3). That is, step 603 compares pixelvalues at same coordinates in the two differential absolute value imagessmoothed at step 602, and composes a pixel value using correspondingpixel values of the original images at a ratio of the compared pixelvalues. In this method, comparing with the method shown in FIG. 5,influence of a blur due to focus deviation is added. However, thismethod is characterized by a smooth transition portion where thein-focus state is switched from image A to image B.Ci,j=(Ai,j×[ΔA]i,j+Bi,j×[ΔB]i,j)/([ΔA]i,j+[ΔB]i,j)  (3)

FIG. 7 is a flowchart showing still another flow of processes forextracting in-focus portions and creating a composite image according tothe present invention. Steps 701 and 702 create differential absolutevalue images, and smooth the images, respectively, as at steps 501 and502 in FIG. 5. Next, step 703 determines which one of the two images isin focus, and creates a composite image. Here, the in-focus evaluationis performed based on the formula (4). That is, step 703 compares pixelvalues at same coordinates in the two differential absolute value imagessmoothed at step 702, and composes a pixel value using correspondingpixel values of the original images at a weighted ratio of the comparedpixel values. This is a method somewhere in between the methodsillustrated in FIG. 5 and FIG. 6. If a weight coefficient K is set to 1,this method is equal to the method shown in FIG. 6, while this methodapproaches the method shown in FIG. 5 if the weight coefficient k is setto a value larger than 1.[ΔA]i,j≧[ΔB]i,jCi,j=(k×Ai,j×[ΔA]i,j+Bi,j×[ΔB]i,j)/(k×[ΔA]i,j+[ΔB]i,j)  (4)[ΔA]i,j<[ΔB]i,jCi,j=(Ai,j×[ΔA]i,j+k×Bi,j×[ΔB]i,j)/([ΔA]i,j+k×[ΔB]i,j)

FIG. 6 and FIG. 7 illustrate composition using two images, andcomposition using n images can be performed by comparing differentialabsolute values or Sobel-filtered pixel values at same coordinates in nimages, and selecting the first and second largest values to apply thesame process to them. Furthermore, although in FIG. 6 and FIG. 7,differential absolute values are used as in-focus evaluation quantities,Sobel-filtered pixel values can be used in the same process flow.

In the above configuration, a two-dimensional image whose every area isin focus, taking local in-focus into account, can be composed by asimple calculating means.

Second Embodiment

FIG. 8 is a flowchart showing a flow of processes in which imageacquisition, extraction of in-focus portions, and creation of acomposite image are performed in parallel according to an embodiment ofthe present invention. Numerical 813 indicates a process in which afocal position, which can be represented by an exciting current, isvaried with time.

Description will be made of processes performed as time elapses, takingprocesses at steps 801 through 812 as examples. An image A1 at step 801is captured at time a1. A differential absolute value image ΔA1 of theimage A1 is created at step 803 before the next image capture at timea2, and an image A2 at step 802 is captured at time a2. Before the nextimage capture at time a3, a differential absolute value image ΔA2 iscreated at step 804, and the differential absolute value image ΔA1 atstep 803 is compared with the differential absolute value image ΔA2 atstep 804, and at step 805, an image ΔGI each of whose pixels is a largerdifferential absolute value is created. At step 806, to prepare for thenext composition, an image S1 each of whose pixels is a correspondingpixel value of the original image of a differential absolute value imagehaving a larger differential absolute value, determined on the basis ofthe image ΔG1 acquired at step 805, is created. Here, a composite imageF1 at step 807 is created based on the image ΔG1 acquired at step 805using the method illustrated in FIGS. 5 through 7.

The composite image F1 is displayed in a display device 112 shown inFIG. 1. Next, an image A3 at step 808 is captured at time a3. Before thenext image capture at time a4, a differential absolute value image ΔA3is created at step 809, and the differential absolute value image ΔA3 iscompared with the image ΔG1 acquired at step 805, and at step 810, animage ΔG2 each of whose pixels is a larger differential absolute valueis created. At step 811, to prepare for the next composition, an imageS2 each of whose pixels is a corresponding pixel value of the originalimage of a differential absolute value image having a largerdifferential absolute value, determined on the basis of the image ΔG2acquired at step 810, is created. Here, a composite image F2 at step 812is created based on the image ΔG2 acquired at step 810, and displayedsubsequently after the composite image F1. That is, a composite image ofthe previously captured images is completed and displayed at the time ofcapturing the next image. By repeating the same processes, imagecapture, composition processing, and indication can be performed inparallel so as to display composite images in real time. Furthermore,parallel processing such as this makes it possible to reduce the controltime it takes to correct a focus, increasing the speed of automaticfocus correction control.

Third Embodiment

FIG. 14 is a diagram showing an indication example for a display device112 for an apparatus according to an embodiment of the presentinvention. This indication example shows a composite image of a contacthole formed in a semiconductor wafer. An apparatus according to thisembodiment has almost the same configuration as that described by use ofFIG. 1, and, therefore, the description provided earlier will not berepeated.

Incidentally, this apparatus embodiment is provided with a pointingdevice (now shown) for moving a cursor 1401 on the display screen of thedisplay device 112. This pointing device is used to select a specificarea on the display screen. This apparatus embodiment has the functionof replacing an area selected by this pointing device with anotherimage. This function will be described by way of example.

The display device 112 shown in FIG. 14 is displaying an image of acontact hole formed in a semiconductor wafer. The above compositionprocesses have been applied to this sample image. When the cursor 1401is placed in a center portion 1402 of the contact hole displayed in thisdisplay device 112 to select this portion, the portion, which is createdby an electron beam having almost the same focal distance as that forthe selected point, that is, a selected area in a specific originalimage among the original images each registered using one of a pluralityof different focuses, is replaced by another image. This replacementprocess is performed based on address data of pixels which areregistered in the above specific original image and have an in-focusevaluation value larger than a predetermined value or almost the samein-focus evaluation value as that for the selected point.

With this arrangement, edges of a contact hole can be made distinct. Forexample, a selected area (in the above example, the center portion 1402of the hole) may be indicated in black so that it is in clear contrastwith the other portions.

This method is effective when edge portions of a contact hole showlittle changes in their brightness. In a scanning electron microscopeused to form a line profile based on image data and measure a patternlength using this line profile, unclear contrast in an edge portioncauses an error in edge-position determination performed based on theline profile. Adoption of an apparatus according to this embodiment ofthe present invention can solve the above technical problem.

Although in the above description, a selected area in an image isreplaced with another image, this apparatus may be configured so thatthe focus of a selected area can be adjusted. Specifically, the cursor1401 is placed in the center portion 1402 of the contact hole in thecomposite image displayed in the display device 112 to select theportion.

Then, the selected area in an original image is replaced with pixelswhich are in a specific original image forming the selected area imageand which have an in-focus evaluation value larger than a predeterminedvalue or almost the same in-focus evaluation value as that for theselected point by the cursor 1401. With this arrangement, it is possibleto perform an operation in which it looks as if to selectively adjustthe focus of a specific portion of a sample image.

In an area whose image is to be replaced, the portion which has almostthe same focus as that for the selected point by the cursor 1401, thatis, a specific image among images each registered using one of aplurality of focuses, replaces another registered image.

Although in the above description, an image of a portion whose focus isalmost equal to that for a selected point is replaced, this should notbe construed restrictively; for example, a means for selecting anarbitrary area in a sample image may be provided, and an image in theselected arbitrary area is replaced based on address data of the area.

Although the above description illustrates an example in which anoperator manually performs operations while observing the display screen112, this should not be construed restrictively; for example, an imagemay be replaced with an image of a specific focus in an automatedprocess.

Fourth Embodiment

FIG. 9 shows indication examples for displaying composite images on areal time basis according to the present invention. An indicationexample 901 displays images composed one after another, in the displaymonitor of a workstation, etc. by dividing the screen to accommodateeach of the composite images so that the process in which the compositeimages are produced can be observed by comparing one image with thenext. The other indication example 902 displays only the latestcomposite image from among the images composed one after another in thedisplay monitor. This embodiment also has a function in which it ispossible to stop the series of processes from acquisition of an image toits indication, by input from a input means 113 connected to a controlcomputer 111 shown in FIG. 1 when an image having a desired in-focusportion has been found, while observing the series of composite images.

With this arrangement, it is possible to eliminate unnecessary electronbeam irradiation that is not related to capturing of an image, andperform automatic focus control of a target portion efficiently and in ashort time.

Fifth Embodiment

FIG. 10 shows an example of measuring a length using a composite imageaccording to the present invention. A composite-image creation functionaccording to the present invention is added to a scanning electronmicroscope having a function of measuring the shape of a semiconductorso that it is possible to measure a shape on a composite image by use ofthese functions.

Furthermore, it is possible to select an image of a specific focus,selectively read pixels indicating an in-focus evaluation value largerthan a predetermined value from the image, and measure a length based onthe pixels. This arrangement makes it possible to, for example,selectively read only the bottom portion image of a contact hole andmeasure a length based on the image so as to eliminate an error inlength measurement due to an erroneous judgment of an edge position ofthe contact hole, resulting in realization of highly accurate lengthmeasurement.

Sixth Embodiment

FIG. 12 is a schematic diagram showing a method for determining theheight difference between two given points in a composite imageaccording to the present invention using the difference between excitingcurrents used when the original images of the pixels at the two givenpoints are captured. To find the height difference between two points(pixels) g1 and g2 in a composite image 1201, it is necessary to checkin which original images 1202 the corresponding pixels existed. When thepoint g1 existed in an original image 2 and the point g2 existed in anoriginal image 5, the height difference between the two points g1 and g2can be calculated from a difference Δd between a focal distance d2corresponding to the exciting current for the original image 2 and afocal distance d5 corresponding to the exciting current for the originalimage 5, using a relationship between an exciting current and a focaldistance shown in FIG. 13.

FIG. 15 shows a GUI screen (Guide User Interface) example for specifyingthe points g1 and g2 in a display device. This GUI screen has a cursor1401 movable by a pointing device and a display column 1501 for lengthmeasurement results, therein. If it is arranged such that the points g1and g2 can be specified by use of the cursor 1401, an operator can, forexample, specify the surface of a sample and the bottom surface of acontact hole while observing the image of the contact hole so that thedepth of the contact hole can be measured.

According to this embodiment, positions to be set as the points g1 andg2 (reference points for depth-direction measurement) can be accuratelyspecified in a two-dimensional image, which makes it possible toaccurately measure a depth-direction dimension of a sample, which isdifficult to determine in a two-dimensional image. Since in the exampleshown in FIG. 15, the point g1 is set to the surface of a sample and thepoint g2 is set to the bottom of a contact hole, the formation depth ofthe contact hole can be accurately measured using the sample surface asa reference level.

Although the above description illustrates an example in which the twopoints g1 and g2 are specified as references for dimensionalmeasurement, this should not be construed restrictively. A point g3 maybe specified in addition to the points g1 and g2. Then, a sequence maybe incorporated to measure the dimensional difference between the pointsg1 and g2, and the dimensional difference between the points g1 and g3so that, for example, the depths of two contact holes can be compared.Since this specific example uses the same g1 as a reference for bothcontact holes, it is possible to accurately compare the formation depthsof the contact holes.

An apparatus according to this embodiment can adopt a decelerationelectric field forming technique in which a negative voltage is appliedto a sample 102 or a sample stage 101 on which the sample is placed toproduce an electric field between the sample or sample stage and anelectron lens 106 which is set to a ground potential so as to reduce theenergy of an irradiation electron beam when it has reached the sample(not shown).

This technique (hereinafter referred to as retarding technique) attainsboth reduction of color aberration by passing an electron beam throughthe electron lens 106 at high acceleration speed and prevention ofcharge-up by reducing the acceleration speed of the electron beam whenit has reached a sample.

In a scanning electron microscope using a retarding technique, anegative voltage is applied to a sample as described above. The focus ofan electron beam can also be adjusted by adjusting this applied negativevoltage. In an embodiment according to the present invention, a negativevoltage applied to a sample may be changed in a stepwise manner, and animage obtained at each step may be stored. In this case, a focaldistance can be decided by the magnitude of the applied negativevoltage.

As described in detail above, an apparatus according to an embodiment ofthe present invention can acquire a sample image which is locally infocus.

Seventh Embodiment

FIG. 18 is a schematic diagram showing an image composing process inwhich an in-focus degree is determined using different types of signalsdetected at the same time according to an embodiment of the presentinvention. Different types of signals that can be detected at the sametime in a scanning electron microscope are secondary electrons andreflection electrons. A general SEM image uses secondary electrons, butreflection electrons are sometimes used to obtain additional informationabout a sample. When a full-focused image is composed using reflectionelectrons, if reflection electron signals are weak and, as a result, theS/N ratio of each reflection electron image having a different focus islow, an image obtained by applying a differential process or a Sobelfilter to a reflection electron image sometimes cannot be used foraccurately performing in-focus determination. In this case, a secondaryelectron image is used for in-focus determination, while a reflectionelectron image is used for image composition. Numerals 1801 and 1802denote a plurality of reflection electron images and a plurality ofsecondary electron images, respectively, captured at the same time bychanging a focus. Therefore, a point g1 in an image 1801 and a point g1in an image 1802 have different signal intensities but are located atthe same position in a sample. Each of images 1803 is obtained byapplying a Sobel filter to one of the secondary electron images 1802.Pixels Sg1 through Sg5 at same coordinates in the plurality of images1803 are compared, and of these pixels, the largest one is detected.Supposing that pixel is the pixel Sg2, a pixel value g21 of a reflectionelectron image acquired at the same time with a pixel value g2 of theoriginal image corresponding to the pixel Sg2 is projected to a pixel atthe same coordinates in a composite image. A reflection electroncomposite image 1803 can be created by applying this process to allcoordinates of the images.

When use of only one type of signals for characteristic quantitycomparison does not produce satisfactory results, another type ofsignals detected at the same time with the former type may also be used.FIG. 19 shows an example of an image composed by comparing also anothertype of signals. FIG. 18 shows an example in which secondary electronimages are also used to compose a reflection electron image. In FIG. 18,reflection electron images are generally used for in-focusdetermination, and when use of reflection electron images for in-focusdetermination cannot produce satisfactory results, secondary electronimages are additionally used. That is, in FIG. 19, an area 1901 isdetermined by in-focus determination using reflection electron images atthe first stage, while an area 1902 is determined by in-focusdetermination using secondary electron images at the second stage sincethe area 1902 cannot be determined using reflection electron images atthe first stage.

FIG. 20 is a schematic diagram showing a scanning electron microscopehaving a plurality of detectors according to the present invention.Components indicated by numerals 2001 through 2014 correspond tocomponents indicated by numerals 101 through 114 in FIG. 1. An electronbeam 2014 is scanned on a sample 2002 by a scanning coil 2005, and aplurality of different types of electrons, for example secondaryelectrons and reflection electrons, emitted from the sample 2002 aredetected by detectors. Secondary electrons are detected by a detector2003, while reflection electrons are detected by a detector 2015. Asignal S1 from the detectors 2003 and 2015 is input to an AD converter2007, which converts the signal into a digital signal S2.

Eighth Embodiment

A charged particle beam apparatus represented by a scanning electronmicroscope, or an optical inspection apparatus, which irradiates lightsuch as laser light onto a sample, scans a beam on a target sample toobtain a pattern image of, for example, a semiconductor, an imagesensor, or a display element. The embodiment described below relates toa technique suitable for properly scanning a sample regardless of itsconcave/convex portions to form a sample image, and inspecting thesample based on the sample image, in a charged particle beam apparatusor an optical inspection apparatus.

A beam scanning inspection apparatus such as a scanning electronmicroscope (hereinafter referred to as SEM) is suitable for measuring orobserving patterns formed on a semiconductor wafer, which has beenbecoming finer. Of SEMs, a length measuring SEM produces a line profilebased on, for example, a contrast obtained by irradiation of an electronbeam onto a sample, or a signal amount of a secondary signal (secondaryelectrons and reflection electrons) generated from the sample to measurepattern dimensions based on the line profile.

Since SEMs thinly converge a beam and irradiate it onto a sample, it isnecessary to properly focus the beam on the sample. Generally, a beam isfocused so that a blur in edges of a sample structure image is minimizedover the entire image.

However, semiconductor wafers recently have been multilayered and havebecome finer, making greater the height difference between the surfaceof a sample and the upper surface of a pattern formed on the sample orthe bottom surface of a contact hole, as well as increasing the aspectratio. As a result, a problem has arisen that the upper surface of apattern and the sample surface have different proper beam focaldistances.

A scanning electron microscope having an automatic focus controlfunction as disclosed in Japanese Laid-Open Patent Publication No.6-89687 (1994) uses a technique to change a focus in a stepwise mannerand determine a proper excitation condition for an object lens based ona detection signal acquired for each focus. This method, however, canacquire only an average focus over the entire sample image, and has theproblem that a portion having a height different from that of the samplesurface, such as a pattern, becomes partially out of focus.

This means that it is not possible to accurately measure the formationwidth of a pattern formed on a semiconductor wafer, etc, and thisproblem has caused reduced measurement accuracy.

An object of this embodiment is to provide a beam scanning inspectionapparatus capable of accurately measuring the formation width of apattern formed on a semiconductor wafer, etc. by solving the aboveproblem.

To accomplish the above object, a beam scanning inspection apparatusaccording to this embodiment forms images of a sample based on signalsobtained by scanning a beam on the sample, said beam scanning inspectionapparatus comprising: a means for changing a focus of said beam in astepwise manner; a storage means for storing a sample image for eachfocus changed by said means; and a means for forming a sample image byoverlapping the sample images stored in the storage means.

This beam scanning inspection apparatus also forms a line profile basedon the overlapped sample image to measure dimensions according to theline profile.

In order to realize high integration density and high operation speed ofsemiconductor devices, it has been demanded to develop finer patternsformed on a semiconductor wafer, devices having a three-dimensionalstructure, and multilayered wiring.

On the other hand, development of finer patterns necessitates highermeasurement accuracy on the inspection apparatus side, while developmentof devices having a three-dimensional structure and multilayered wiringfurther increases the aspect ratio (pattern height/pattern width) of apattern to be measured. What this trend means to length measuring SEMsis that it is necessary to realize a higher resolution for increasingmeasurement accuracy and an increased focal depth for enablingobservation of high aspect patterns (large-height-difference pattern) atthe same time.

However, a resolution R and a focal depth DOF are proportional to eachother as indicated by the following formula. A Focal depth decreases(becomes shallower) as a resolution is enhanced (becomes smaller) Thatis, their effects work against each other.R∝dDOF∝d/sin α (∝R)

d: diameter of electron beam, α: half aperture angle of electron beam

Therefore, when a fine and large-height-difference pattern is measuredunder high resolution conditions (extremely thin electron beam),focusing on the upper surface of the pattern blurs the surface of thesubstrate, making it impossible to measure pattern edges on thesubstrate surface (desired pattern width) with high length measurementaccuracy.

On the other hand, considering the current focusing technique, it isvery difficult to control an electron beam so that it is always focusedon the substrate surface.

An object of this apparatus embodiment is to attain both enhancement ofresolution and increasing of a focal depth which are mutuallycontradictory as described above.

To acquire high resolution, an electron beam diameter is decreased byincreasing the reduction ratios of a converging lens and an object lens.Generally, the aperture angle (2α) of an electron beam incident on asample surface increases as the reduction ratio is increased. As theaperture angle (2α) increases, an increase in the diameter of theelectron beam (2α·ΔF) due to a focus deviation (ΔF) becomes larger.Observation with higher resolution is possible with a smaller electronbeam diameter d on a focal surface since an electron beam of a smallerdiameter is irradiated to an object on the focal surface. On the otherhand, the image of an object placed apart from the focal surface,however little it is apart, becomes significantly blurred since theelectron beam diameter (d+2α·ΔF) becomes larger.

Considering this problem, in order to attain both a large focal depthand a high resolution, this embodiment stores images captured with alarge half aperture angle α and several focal positions matching theheight of a pattern, and forms a sample image by overlapping thesestored images. The principle is shown in FIGS. 22(A) to 22(D).

FIG. 22(C) shows a line profile obtained when a beam focused on theupper surface of a pattern (in the left in FIG. 22 (A)) is scannedacross the pattern. The edge profile on the upper surface of the patternis distinct, while the edge profile on the substrate surface is notdistinct. When a beam focused on the substrate surface is used, on theother hand, a line profile shown in FIG. 22(B) is produced in which theedges on the substrate surface is distinct but the edges on the uppersurface is not.

Overlapping of line profiles in FIGS. 22(B) and 22(C) produces a lineprofile having distinct edges both on the upper surface and thesubstrate surface as indicated by a solid line in FIG. 22(D). A brokenline in FIG. 22(D) indicates an ideal line profile obtained when anideal point beam (all portions in focus) is scanned across the pattern.As seen from the figure, the overlapped line profile indicated by thesolid line in FIG. 22(D) is close to the ideal signal intensitydistribution, compared with those in FIG. 22(B) and FIG. 22(C). That is,use of a overlapped profile makes it possible to accurately measure thedimension of a large height-difference pattern even with its focalposition deviated.

As described above, the effect of an increased focal depth byoverlapping images becomes more distinct with higher resolution.Furthermore, with a larger electron beam half aperture angle a, that is,with a smaller electron beam diameter, the image of an object at a focalposition becomes clearer, whereas the image of an object apart from thefocal position, however little apart, becomes more unclear (only abackground contributing to only increasing of brightness of the entireimage) This means that an overlapped image has a higher resolution.

Utilizing the above principle, this embodiment comprises: a means forrepeating setting of a beam focal position, and formation and capture ofa predetermined number of frame images; and a means for overlapping theplurality of frame images acquired by the above means to form a sampleimage.

With the above arrangement, both a high resolution and an increasedfocal depth can be attained. As a result, it is possible to accuratelymeasure the dimension of a fine and large-height-difference patternregardless of accuracy of a focal position.

It should be noted that changing a focal position on a frame basis makesthe control operation easy, reducing redundant time in measurement.

An apparatus according to this embodiment will be described in detailbelow using FIG. 21. Although the apparatus will be described withreference to a length measuring SEM used to measure the dimension of apattern formed on a semiconductor wafer, etc., this should not beconstrued restrictively. A scanning ion microscope using an ion beam, oran optical microscope which forms a sample image by scanning laser lighton the sample, may be used.

An electron beam 3002 emitted from an electron gun 3001 is thinlyconverged by a converging lens 3003 and an object lens 3004 after thebeam is accelerated, and is focused on a sample wafer 3005. Theconverging lens is used to control the electron beam current value,while the object lens is used to adjust the focal position.

The electron beam 3002 that has been focused on the wafer is deflectedby a deflector 3006 so that it is scanned on the wafer surfacetwo-dimensionally or one-dimensionally. Part of the wafer irradiatedwith the electron beam 3002, in turn, emits secondary electrons 3007.The secondary electrons 3007 are detected by a secondary electrondetector 3008 and converted into an electric signal. It should be notedthat even though the following description assumes and thereby explainsthat the secondary electrons 3007 are detected, reflection electrons maybe detected instead of, or in addition to the secondary electrons 3007.

The electric signal is subjected to signal processing such as A/Dconversion in a signal processing unit 3014. The image signal subjectedto signal processing is stored in a memory unit 3015, which is a storagemedium, and used to apply intensity modulation or Y-modulation to adisplay 3009. The display 3009 is scanned in synchronization withscanning of an electron beam 3002 on the wafer surface so that a sampleimage is formed in the display. When the display is scannedtwo-dimensionally with intensity modulation applied, an image isdisplayed, while when the display is scanned one-dimensionally withY-modulation applied, a line profile is drawn.

The sample image (an image and/or a line profile) is used to measure apattern dimension as follows:

(1) First, an image is formed, and positioning of a pattern to bemeasured and focusing are performed.

(2) Next, a line profile is formed by scanning an electron beam acrossthe pattern to be measured one-dimensionally in such a direction that adesired dimension can be acquired.

(3) Then, the pattern edges are determined from the line profileaccording to a predetermined pattern edge determination algorithm.

(4) Then, the dimension of the pattern to be measured is calculated fromthe distance between the pattern edges.

(5) Finally, the calculated value acquired is output as a dimensionalmeasurement result.

A threshold method or a linear approximation method is generally used asa pattern edge determination algorithm. In the threshold method, a lineprofile is cut by a given threshold level, and the intersection pointsof the line profile and the threshold level are determined as patternedges.

By setting the threshold level to 50%, a measurement result close to theactual dimension can be acquired. In the linear approximation method,changes in a line profile in pattern edge portions are linearlyapproximated, and the intersection points of this approximated line andthe base line of the line profile are determined as pattern edges.

A pattern measurement accuracy (length measurement accuracy) isinversely proportional to the diameter of an electron beam as a firstapproximation. Decreasing an electron beam diameter can reducevariations in length measurement values. on the other hand, decreasingan electron beam diameter also reduces an electron beam current.Specifically, an electron beam current is proportional to the square ofan electron beam diameter. If a beam current is excessively reduced todecrease an electron beam diameter, the S/N ratio of a sample image willbe lowered.

A large reduction in the S/N ratio deteriorates image quality, that is,resolution, and reduces length measurement accuracy. That is, to achievehigh length measurement accuracy, it is necessary to attain both areduced electron beam diameter and a sufficient S/N ratio (which lead tohigh resolution).

To satisfy these requirements, this embodiment employs a method in whicha plurality of frames are overlapped to form a sample image. Theprocesses employed in this method are performed in the following order.

(1) A sample image is formed using an electron beam of a small diameter.(This is called a frame image. The image quality is not good because theS/N ratio is low.)

(2) Formation of a frame image is repeated a plurality of times, forexample, 20 times.)

(3) Twenty frame images obtained in (2) are overlapped to form a sampleimage of a sufficient resolution (an image which is captured with asmall electron beam diameter and has a high SIN ratio).

(4) The sample image acquired in (3) is processed to calculate patterndimensions.

In this embodiment, the following procedure is used to measure a patterndimension.

A wafer 3005 to be measured is extracted from a wafer cassette 3010 andis prealigned. Prealignment is an operation performed to orient thedirection of a wafer using an orientation flat and a notch formed on thewafer as references.

After prealignment, a wafer number formed on the wafer 3005 is read by awafer number reader (not shown). A wafer number is specific to eachwafer. A recipe previously registered for this wafer is read using theread wafer number as a key. After this, operations are performedaccording to this recipe automatically or semi-automatically.

After the recipe is read, the wafer 3005 is transferred onto a X-Y stage3012 in a sample chamber 3011, which is kept vacuous, and is loadedthere. The wafer 3005 fitted on the X-Y stage 3012 is aligned using anoptical microscope 3013 fitted on the upper surface of the samplechamber 3011 and an alignment pattern formed on the wafer 3005.

The alignment is performed using the alignment pattern formed on thewafer to correct the position coordinate system on the X-Y stage and thepattern position coordinate system in the wafer. An optical microscopeimage acquired by magnifying the alignment pattern a few hundreds timesis compared with an alignment pattern reference image registered in thememory unit 3015, and correct the stage position coordinates to exactlyalign the visual field of the optical microscope image with that of thereference image.

After the alignment, the visual field is moved to the position of apredetermined pattern to be measured by use of the X-Y stage 3012 and adeflection coil 3006, and the pattern to be measured is positioned.Then, after the focal position is set with the current visual fieldposition, a sample image of the pattern to be measured is formed usingthe frame image overlapping method as described above and stored in thememory unit 3015.

Focusing is performed by generally observing the signal intensitydistribution (a line profile) of a sample image, and adjusting the focusto a position at which the line profile is considered to be mostdistinct. Then, using a sample image read from the memory unit 3015, adimension measurement unit 3016 calculates the dimension of the patternto be measured according to a predetermined pattern edge determinationalgorithm.

An apparatus according to this embodiment performs the followingprocesses at the time of forming a sample image. Utilizing thecharacteristic that the focal distance of a magnetic lens is almostinversely proportional to the square of the exciting current, a controlunit 3017 in FIG. 21 changes the focal position of an electron beam foreach predetermined number of frames by controlling the exciting currentfor the object lens 3004.

For example, within a predetermined range of focal distance set so thatit includes a focal position acquired using the above-mentioned focusingmethod, the exciting current is changed from the largest value to asmaller value by steps (that is, the focal position is changed from theneighborhood of the upper surface of a pattern to the substrate surfaceby steps) to acquire and store frame images like four frames, sixframes, eight frames, ten frames, twelve frames, and so on.

Here, the number of frames captured at one step is increased as thesubstrate is approached, considering the fact that the secondaryelectron signal amount is reduced at positions closer to the substratesince electrons emitted from the substrate are obstructed by the sidewalls of a pattern. This makes it possible to acquire an image havinguniform brightness over the entire sample and a high resolution.

Next, the stored frame images are read out and overlapped to form asample image after they are passed through a high pass filter to cut offlow-space-frequency components. Frame images captured at positionsupwardly away from the pattern surface or downwardly away from thesubstrate surface have almost uniform brightness (image composed of onlylow-space-frequency components) since the electron beam comes greatlyout of focus there, and, therefore, overlapping the frame images withtheir low-space-frequency components cut off does not deteriorate theresolution of the sample image.

These frame image acquisition conditions are registered in the memoryunit 3015 as a recipe. A recipe specifies a measuring procedure ormeasuring conditions to automatically or semi-automatically performmeasuring operations.

Incidentally, cutoff of the low-space-frequency components may beperformed before forming or storing frame images, instead of beingperformed as a preprocess for overlapping of frame images. Either way, asample image having more distinct pattern edges can be acquired sincethe low-space-frequency components are cut off by a high pass filter,making it possible to remove signal components originated from objectsapart from the focal position.

A line profile is formed based on this sample image. Then, patternlengths are measured based on the distance between edges of the lineprofile. In this apparatus embodiment, these processes are performed bythe control unit 3017.

Then, the operations after alignment are repeated for each ofpredetermined measurement positions in the wafer to measure lengths.This completes the measurement of one wafer. If there are a plurality ofwafers to be measured left in the wafer cassette, the next wafer istaken out and measured according to the above procedure, one afteranother. The dimensional measurement results are output together withmeasurement position coordinate data and images of the measurementpositions, and registered with a database (not shown) for futureanalysis.

Even though this embodiment controls the exciting current for the objectlens to change a focal position, the height of the stage may be changedinstead for that purpose.

In the case of an apparatus using a retarding technique in which anegative voltage is applied to a sample to form a deceleration electricfield for an electron beam so as to reduce damage to the sample andcharge-up caused by a highly accelerated electron beam, the negativevoltage applied to the sample may be changed to adjust the focus of theelectron beam.

For an insulator sample whose charge-up takes time to saturate, an imageof the sample may be captured after the sample is irradiated with anelectron beam for a predetermined period of time. This pre-irradiationoperation can be registered as a recipe.

Even though this apparatus embodiment uses an electron beam as a probefor pattern dimensional measurement, an ion beam may be used instead.Also, even though this apparatus embodiment uses one probe, amulti-probe method may be used to form an image. Furthermore, thisembodiment illustrates observation of a semiconductor wafer, but a waferfor an image sensor or a display element, or a sample other than anywafers may be observed.

This apparatus embodiment makes it possible to attain both a highresolution and an increased focal depth. As a result, a sample capturedat a high resolution is more distinctly observed. Furthermore, a lengthmeasuring SEM according to this embodiment can accurately measure thedimension of a fine and large-height-difference pattern.

Ninth Embodiment

As described below, this embodiment relates to an apparatus suitable forreducing mixing of noise during the above-mentioned image composition.

Techniques for observing a three-dimensional structure using an electronmicroscope image are disclosed in Japanese Laid-Open Patent PublicationsNos. 5-128989 (1993) and 5-299048 (1993), which are described earlier,and 9-92195 (1997).

A conventional technique for evaluating a noise amount is described inJohn Immerkaer's “Fast Noise Variance Estimation”, Computer Vision andImage Understanding, Vol. 64, No. 2, 1996, p. 300-302.

As semiconductor wafers have been multilayered, their sample has taken ashape extending three-dimensionally, and furthermore, since enhancedresolution of electron microscopes has decreased focal depth, the needfor observing a three-dimensional structure is increasing. On the otherhand, although conventional techniques for observing a three-dimensionalstructure using an image captured by an electron microscope aredisclosed in Japanese Laid-Open Patent Publications Nos. 5-128989(1993), which observes a three-dimensional image, and 5-299048 (1993),which uses contour lines and bird's eye views, these techniques are notsuitable for forming a two-dimensional image. Compared with thesetechniques, a full-focused image composition technique in which aplurality of images each having a different portion in focus are used tocompose a two-dimensional image whose every portion is in focus, issimple and easy to use, and expected to be applied to electronmicroscope images. However, since an electron microscope image has alarge amount of noise, it is necessary to give consideration to noise.Since an optically captured image includes relatively little noise, theconventional full-focused image composition technique for optical imageshas given no consideration to noise. When an electron microscope imageis composed using the conventional method, some artifacts occur,producing an unnatural full-focused image.

An object of this embodiment is to provide a method and an apparatus forcomposing a full-focused image which looks natural and does not looklike an artificially composed image even from input images having muchnoise.

Description will be made of a composite image generating methodaccording to this embodiment or an apparatus using it, said apparatuscomprising a plurality of full-focused image composing means forperforming a series of steps included in said method, said series ofsteps comprising the steps of: reducing noise of a plurality of inputimages each read with a different focus; evaluating noise amounts of thenoise-reduced images whose noise has been reduced and evaluatingin-focus degrees of said noise-reduced images to calculate signal changeamount evaluation values; generating a maximum signal change amountevaluation value and composition information based on the calculatedsignal change amount evaluation values; and generating favorablenessdegree information by determining a noise influence degree using saidmaximum signal change amount evaluation value and said noise amountevaluation values; and said method further comprising a step of:generating a composite image based on a plurality of pieces of saidfavorableness degree information and a plurality of pieces of saidcomposition information generated by a plurality of said full-focusedimage composing means.

FIG. 23 is a diagram showing the concept of full-focused imagecomposition according to the present invention. Reference numerals 4110,4120, and 4130 indicate a plurality of input images captured by changinga focus; 4140 denotes signal change amount evaluation; 4141, 4142, and4143 are signal change amounts corresponding to the input images 4110,4120, and 4130, respectively; 4150 indicates a composite image; and 4160indicates a depth image. Select a corresponding pixel in each of theinput images 4110, 4120, and 4130 and suppose that the pixel of theimage 4110 and its neighborhood are blurred while the pixel of the image4120 is in focus, and the pixel of the image 4130 and its neighborhoodare also blurred. Signals from the neighborhood of a blurred pixel showlittle changes while signals from the neighborhood of a pixel in focusshow large changes. According to evaluation of those signal changeamounts by the signal change amount evaluation 4140, the signal changeamounts are represented by bars 4141, 4142, and 4143 shown in FIG. 23,indicating that the signal change amount 4142 of the image 4120 in focushas the largest value. Therefore, a pixel of the input image 4120corresponding to the bar 4142 showing the largest signal change amountis set as a pixel at the corresponding position in a composite image4150. Depth information for the image 4120 corresponding to the bar 4142showing the largest signal change amount is set as a pixel at thecorresponding position in a depth-image 4160. Even though the focaldistance of an input image is preferably used as a pixel value toindicate depth information since the actual depth of an imaged object isdirectly given, another method may be employed. For example, the inputimages 4110, 4120, and 4130 may be numbered such as #1, #2, and #3 to beused as depth information. In short, it is only necessary to make surewhich input image has been selected. Repeating the above operation forall pixels composes the composite image 4150 whose every portion is infocus.

Tenth Embodiment

FIG. 24 shows a tenth embodiment according to the present invention.

Input images 4200 are processed in a preprocess 4210, and then fed tofull-focused image composing means 4220 and 4230, which composedifferent full-focused images and generate sets of compositioninformation and favorableness degree information 4221 and 4231,respectively. Based on the sets of composition information andfavorableness degree information 4221 and 4231, a composing means 4240composes a composite image 4250 and a depth image 4260 by selecting eachpixel most suitable for the purpose of the composition fromcorresponding pixels of input images 4211 after the preprocess. Asdescribed above, a plurality of full-focused composite images can beused to compose a more favorable composite image by connecting eachfavorable pixel selected from one of these full-focused compositeimages. Although this example uses two full-focused image composingmeans, the example can easily be extended to employ three or morefull-focused image composing means.

Eleventh Embodiment

FIG. 25 shows an eleventh embodiment according to the present invention.This embodiment discloses a specific configuration of the tenthembodiment by limiting the meaning of the term “favorable” used indescription of the tenth embodiment to mean “unsusceptible to noiseinfluence”.

Since a full-focused image composing means 4230 has the sameconfiguration as that for a full-focused image composing means 4220, thefull-focused image composing means 4220 will be mainly described. Inputimages are preprocessed by a preprocessing means 4210, and then thenoise of the input images is reduced by a noise reducing means 4300.Specific examples of the noise reducing means include a noise reductionfilter and smoothing reduction. Here, smoothing reduction example willbe described. A noise amount evaluating means 4310 calculates noiseamount evaluation values 4311 of noise-reduced images 4301, while asignal change amount evaluating means 4320 calculates signal changeamount evaluation values 4321, which indicate in-focus degrees, from thenoise-reduced images 4301. A focus determination means 4340 generates amaximum signal change amount value 4342 and composition information 4341using the signal change amount evaluation values 4321. The maximumsignal change amount value 4342 is fed to a noise determination means4330, which outputs favorableness degree information 4331 indicating thedegree of noise influence. The full-focused image composing means 4230operates similarly as the full-focused image composing means 4220.However, if a noise reducing means 4350 is set so that it provideslarger smoothing reduction than the noise reducing means 4300 does, theprocessing results from the full-focused image composing means 4230shows larger noise reduction effect but reduced space resolution,compared with those from the full-focused image composing means 4220.That is, while the full-focused image composing means 4220 provides animage that does not have many favorable portions free from noiseinfluence but has a high space resolution, the full-focused imagecomposing means 4230 provides an image that has many favorable portionsfree from noise influence but has a low space resolution. A composingmeans 4240 composes a composite image 4250 that has many favorableportions free from noise influence and a high space resolution, byselecting each pixel subjected to less noise influence from one of theimages to compose a composite image having little noise. Although thisexample uses two full-focused image composing means, the example caneasily be extended to employ three or more full-focused image composingmeans.

FIG. 26 shows a configuration of a focus determination means 4340.

A maximum value storage means 4410 currently stores a maximum signalchange amount value 4411 calculated up to the last image. A maximumvalue calculating means 4420 compares the maximum signal change amountvalue 4411 calculated up to the last image against a signal changeamount 4321 of the current image, and selects the larger one as amaximum signal change amount value 4342 for up to the current image toupdate the maximum value stored in the maximum value storage means 4410with the selected value. A subtracting means 4430, on the other hand,calculates the difference between the maximum signal change amount value4411 calculated up to the last image and the signal change amount 4321of the current image, and a binarizing means 4440 determines whether thedifference is larger than zero. That is, when the signal change amount4321 is denoted as f and the maximum signal change amount value 4411calculated up to the last image is denoted as fmax and compositioninformation 4341 is represented as g,if f>fmax, g=1if f<fmax, g=0Therefore, when the signal change amount 4321 is larger than the maximumsignal change amount value 4411 calculated up to the last image, g=1.That is, when a pixel of the current image should be selected as thatfor a composite image, its composition information g is set to 1.

FIG. 27 shows a configuration of a noise determination means 4330. Thenoise determination means 4330 receives the noise amount evaluationvalues 4311 and the maximum signal change amount value 4342, and outputsthe favorableness degree information 4331. When the favorableness degreeinformation 4331 is denoted as v and the maximum signal change amountvalue 4342 is denoted as fmax and a noise amount evaluation value 4311is represented as N, if the maximum signal change amount value fmax islarger than the noise amount evaluation value N, the favorablenessdegree information v is set to 1 since there is little noise influence,which makes the favorableness degree high. When the maximum signalchange amount value fmax is smaller than the noise amount evaluationvalue N, the favorableness degree information v is set to 0 since thereis large noise influence, which makes the favorableness degree low. Itshould be noted that a noise amount evaluation value 4311 is multipliedby a constant K to change its scale so that the noise amount evaluationvalue 4311 can be appropriately compared with the maximum signal changeamount value 4342. The above description of the noise determinationmeans 4330 is expressed as follows:When f(x,y)>K*n, v(x,y)=1When f(x,y)<K*n, v(x,y)=0The favorableness degree information 4331 to be output is set to 1 whenthere is little noise influence, which makes the favorableness degreehigh, while it is set to 0 when there is large noise influence, whichmakes the favorableness degree low.

FIG. 28 shows a configuration of the composing means 4240. The composingmeans 4240 receives pieces of composition information 4341 and 4343,pieces of favorableness degree information 4331 and 4333, andpreprocessed input images 4211, and outputs the composite image 4250 anda depth image 4260. The pieces of favorableness degree information 4331and 4333 are information used to show noise influence by indicating aportion subjected to little noise influence as 1 (white) and a portionsubjected to large noise influence as 0 (black). Referring to FIG. 25,if it is arranged such that a noise reducing means B4350 provides largersmoothing reduction than that provided by a noise reducing means A4300,portions subjected to little noise influence, which is indicated by avalue of I (white), in the favorableness degree information 4333 shouldbe more extended than portions subjected to little noise influence,which is also indicated by a value of 1 (white), in the favorablenessdegree information 4331. Supposing that the pieces of compositioninformation 4341 and 4343 have shapes as indicated in the figure, forportions in which the favorableness degree information 4331 is 1, thecomposition information 4341 is preferably used, while for portions inwhich the favorableness degree information 4331 is 0 and thefavorableness degree information 4333 is 1, the composition information4343 is preferably used. Furthermore, even though portions in which boththe favorableness degree information 4331 and the favorableness degreeinformation 4333 are 0 provide no reliable information, thefavorableness degree information 4333 is arbitrarily used since eitherone or the other should be selected. Final composition information 4600composed of only portions subjected to little noise influence is shownin the figure. Portions of the composite image 4250 in which the finalcomposition information 4600 is 1 are updated with pixels of thepreprocessed input images 4211, while portions of the composite image4250 in which the final composition information 4600 is 0 are leftunchanged without updating the original pixels. On the other hand,portions of the depth image 4260 in which the final compositioninformation 4600 is 1 are updated with depth information of thepreprocessed input images 4211, while portions of the depth image 4260in which the final composition information 4600 is 0 are left unchangedwithout updating the original pixels of the depth image 4260. Thus,optimum composition concerning noise reduction is attained by combiningportions subjected to little noise influence with their high spaceresolution unchanged and portions subjected to large noise influencewith their space resolution decreased to reduce their noise.

FIG. 29 is a flowchart showing processes of software realizing the tenthembodiment. A series of steps such as a noise reducing step, a noiseevaluating step, a signal change amount evaluating step, a noisedetermination step, a focus determination step, and a noise reductionchanging step are repeated each time the noise reduction amount ischanged at the noise reduction changing step. Then, after composition isperformed, the next target image is loaded at a step which determinesthe existence of a next image, and the entire series of steps arerepeated until there is no target image left.

That is, even though full-focused image composition means 4220 and 4230are separately shown in a configuration diagram such as FIG. 25,full-focused image composition means 4220 and 4230 can be performed byexecuting a module with its repetition condition changed for each ofthem in a flowchart when they are realized by software. Even in such acase, however, the existence of a plurality of full-focused imagecomposition means must be considered since the module is executedrepeatedly.

FIG. 30 shows the first configuration example of a preprocessing means4210. When input images 4201 and 4202 are obtained by imaging a targetobject at positions that are shifted from each other, performingfull-focused image composition of pixels at a same position in the inputimages 4201 and 4202 does not produce proper results. Therefore, to copewith this problem, for example, an area 4801 of an arbitrary size isselected from the center portion of the input image 4201 as a template,and template matching is performed with the input image 4202.

As a result, if an area 4802 is matched, the areas 4801 and 4802 arecompletely overlapped to select the overlapping square area (an AND area4803) of the input images 4201 and 4202, that is, portions of the inputimages 4201 and 4202 that are not overlapped are removed to produceposition-matched input images 4804 and 4805. Using the position-matchedinput images 4804 and 4805 as input images, full-focused imagecomposition with properly matched pixels can be performed. Although thisexample uses two input images, the example can be easily extended toemploy three or more input images.

FIG. 31 shows the second configuration example of the preprocessingmeans 4210. As in the case shown by FIG. 30, when input images 4201 and4202 are obtained by imaging a target object at positions that areshifted from each other, performing full-focused image composition ofpixels at a same position in the input images 4201 and 4202 does notproduce proper results. Therefore, to cope with this problem, forexample, an area 4801 of an arbitrary size is selected from the centerportion of the input image 4201 as a template, and template matching isperformed with the input image 4202. As a result, if an area 4802 ismatched, the areas 4801 and 4802 are completely overlapped to acquirethe square area (an OR area 4903) that includes the input image 4201 or4202 or both, and the portions of the OR area 4903 that do not overlapthe input image 4201 are added to the input image 4201 by giving a valueof 0 or the average value of the input image to each pixel of theportions to produce a position-matched input image 4904, while theportions of the OR area 4903 that do not overlap the input image 4202are added to the input image 4202 by giving a value of 0 or the averagevalue of the input image to each pixel of the portions to produce aposition-matched input image 4905. Using the position-matched inputimages 4904 and 4905 as input images, full-focused image compositionwith properly matched pixels can be performed. Although this exampleuses two input images, the example can be easily extended to employthree or more input images.

FIG. 32 shows the third configuration example of the preprocessing means4210. When the intensity levels of an input image f1 and an input imagef2 do not coincide, that is, the average value a of the pixel values ofthe input image f1 does not coincide with the average value b of thepixel values of the input image f2, if full-focused image composition isperformed using those input images as they are, borders between portionsmade up of pixels of the input image f1 and portions made up of pixelsof the input image f2 are distinct, producing an unsatisfactory result.Therefore, the intensity levels of both the input images f1 and f2 areconverted so that the average values of the pixel values of both imagesare equal to the same average value c.f1(x,y)=f1(x,y)+c−af2(x,y)=f2(x,y)+c−bSince the above conversion makes the average pixel values of both theinput image f1 and the input image f2 coincide with the value c, whenfull-focused image composition is performed using those input images,borders between portions made up of pixels of the input image f1 andportions made up of pixels of the input image f2 are not distinct,producing a satisfactory result. Although this example uses two inputimages, the example can be easily extended to employ three or more inputimages.

Twelfth Embodiment

FIG. 33 shows a twelfth embodiment according to the present invention.

Input images are preprocessed by a preprocessing means 4210, and thenthe noise of the input images is reduced by a noise reducing means 4300.Specific examples of the noise reducing means include a noise reductionfilter and smoothing reduction. A noise amount evaluating means 4310calculates noise amount evaluation values 4311 of noise-reduced images4301, while a signal change amount evaluating means 4320 calculatessignal change amount evaluation values 4321, which indicate in-focusdegrees, from the noise-reduced images 4301. A focus determination means4340 generates composition information 4341 using signal change amountevaluation values 4321. A composing means 5240 composes a compositeimage 4250 and a depth image 4260 from the composition information 4341.

Thus, in full-focused image composition, by reducing noise of the inputimages by the noise reducing means 4300 before the signal change amountevaluating means 4320 evaluates signal change amounts, it is possible tocompose the composite image 4250 subjected to little noise influence.

Thirteenth Embodiment

FIG. 34 shows a thirteenth embodiment of the present invention.

A full-focused image composing means 4230 consists of only a defaultcomposition information generating means 5301. Input images arepreprocessed by a preprocessing means 4210, and then the noise of theinput images is reduced by a noise reducing means 4300. A noise amountevaluating means 4311 calculates noise amount evaluation values 4311 ofnoise-reduced images 4301, while a signal change amount evaluating meansA4320 calculates signal change amount evaluation values 4321, whichindicate in-focus degrees, from the noise-reduced images 4301. A focusdetermination means 4340 generates a maximum signal change amount value4342 and composition information 4341 using the signal change amountevaluation values 4321. The maximum signal change amount value 4342 isfed to a noise determination means 4330, which outputs favorablenessdegree information 4331 indicating the degree of noise influence. Thedefault composition information generating means 5301 outputscomposition information 4343 so as to set a value of 1 to all pixelswhen a default image is input, and set a value of 0 to all pixels whenan image other than a default image is input, in order to always selecta fixed default image.

A full-focused image composing means 4220 forms an image which does nothave many favorable portions subjected to little noise influence but hasa high space resolution, while the full-focused image composing means4230 selects a fixed default image. A composing means 4240 composes acomposite image 4250 that has many favorable portions subjected tolittle noise influence and a high space resolution by selecting pixelsof the image acquired from the full-focused image composition means 4220for portions subjected to little noise influence, and selecting pixelsof the default image for portions subjected to noise influence.

Fourteenth Embodiment

FIG. 35 shows a fourteenth embodiment according to the presentinvention. A full-focused image composing means 5110 receives an inputimage 4201, a maximum signal change amount value image 5101, a composedimage 5102, and a depth image 5103, and outputs a maximum signal changeamount value image 5111, a composed image 5112, and a depth image 5113.The maximum signal change amount value image 5101 corresponds to amaximum signal change amount value fmax 4411 in FIG. 26, the composedimage 5102 corresponds to a composite image 4250 in FIG. 24, and thedepth image 5103 corresponds to a depth image 4260. Full-focused imagecomposing means 5120 and 5130 receives and outputs the same images asthose received and output by the full-focused image composing means5110. Next, the operation of this embodiment will be described below.The full-focused image composing means 5110 checks a signal changeamount of the input image 4201, and compares it with the maximum signalchange amount value image 5101 to update the composed image 5102 and thedepth image 5103 to produce a composed image 5112 and a depth image5113, respectively. Next, the full-focused image composing means 5120checks a signal change amount of an input image 4202, and compares itwith a maximum signal change amount value image 5111 to update thecomposed image 5112 and the depth image 5113 to produce a composed image5122 and a depth image 5123, respectively. Furthermore, the full-focusedimage composing means 5130 checks a signal change amount of the inputimage 4203, and compares it with the maximum signal change amount valueimage 5121 to update the composed image 5122 and the depth image 5123 toproduce a composed image 5132 and a depth image 5133, respectively. In aconfiguration as described above in which input images are fed one byone to generate the current composition results by adding the currentinput image to the last composition results, it is possible to performan input process, in which a next input image is fed, and a composingprocess, in which a full-focused composition process is performed on aninput image, in parallel so that the full-focused image composing means5110, 5120, and 5130 can be included in the input processes, in whichinput images are input. To process ten input images, for example, theabove arrangement can finish it in much shorter time than an arrangementin which all of the ten input images are input before they are subjectedto the full-focused image composition process.

As described above, the fourteenth embodiment according to the presentinvention can provide a method and an apparatus for composing afull-focused image which looks natural and does not look like anartificially composed image even from input images having much noise.

Fifteenth Embodiment

This embodiment, as described below, relates to a technique to form acomposite image based on a plurality of images each having a differentfocus as described so far, and particularly to a technique to suitablychange a focus when acquiring a plurality of images.

Scanning electron microscopes generally employ a magnification variablefrom about a few tens to one million times, thereby having a dynamicrange greatly different from that of optical microscopes. To compose animage. having a large focal depth from a plurality of images each havinga different focal position in an observation apparatus such as this, itis important to optimally control a focus shift amount between images.

To clarify problems to be solved by this embodiment, description will bemade of a relationship between image forming conditions and focal depthwith reference to FIGS. 36 and 37. Both FIGS. 36 and 37 show arelationship between an observation magnification and a range of focusin which a beam can be focused (focal depth) when n scanned image isformed under constant focal conditions. FIG. 36 shows two graphs formedat the same acceleration voltage but different device resolutions: graphA shows a focal depth at a low resolution and graph B shows a focaldepth at a high resolution. FIG. 37 also shows two graphs formed at thesame beam resolution but different acceleration voltages: graph A showsa focal depth at a high acceleration voltage and graph B shows a focaldepth at a low acceleration voltage. To acquire an image having a focaldepth larger than those indicated by a graph under each image formingcondition, it is necessary to compose an image by capturing a pluralityof images each having a different focal position (focal condition). Asshown in FIGS. 36 and 37, a focal depth largely depends on not onlyobservation magnification but also beam conditions such as the beamresolution and acceleration voltage. Furthermore, a focal depth dependson the number of pixels or a pixel size used to form an image. That is,since the size of a pixel becomes smaller as the number of pixels isincreased, the allowable value of a focus displacement of a beamregarded as being in focus also becomes smaller, decreasing the focaldepth. Therefore, it is important to capture scanned images whilecalculating an optimal focus shift amount according to these imageforming conditions including observation magnification. For example, tocompose an image of a high magnification, it is necessary to captureimages while applying a minute focus shift amount to them. In the caseof composition of an image of a low magnification, however, this focusshift amount turns out to be too small.

Thus, when a fixed focus shift amount is used to capture a plurality ofimages to reconstruct an image, a sufficiently extended focal depthcannot be effected if the shift is applied to the entire magnificationrange of a scanning electron microscope. The above conventionaltechnique has given no consideration to this problem and, therefore, toacquire a given focal depth, it is necessary to capture more images thanare necessary. On the other hand, when a plurality of images arecaptured to extend a focal depth, a same portion of a sample isrepeatedly scanned. Therefore, as the number of images used forcomposition is increased, a sample is damaged by a beam, causing theproblem that it is not possible to obtain a composite image properlyreflecting the characteristics of the sample. To cope with this problem,it is necessary to restrict the number of images used for extension of afocal depth to a minimum.

An object of this embodiment is to provide an electron beam apparatuscapable of producing the maximum extension effect of a focal depth usinga minimum of captured images by capturing a plurality of images with anoptimal focus shift amount according to image forming conditions such asdevice parameters and observation conditions.

To solve the above problem, this embodiment comprises a means fordetermining an optimal focus shift amount based on image formingconditions; a means for setting a focal depth necessary for a compositeimage; and a means for determining the minimum number of images to becaptured necessary for satisfying the set focal depth value provided bythe above means.

A focal depth fd of a scanned image under a constant focal conditionwith a low observation magnification is expressed by the followingformula:fd=A1×(dpix/M)×R×√{square root over ( )}Vacc  (1)Where A1 denotes a constant; dpix denotes a pixel size; M represents anobservation magnification; R represents a beam resolution (a resolutiondecided by a beam diameter); and Vacc indicates an acceleration voltage.With a higher observation magnification, the resolution of a scannedimage becomes restricted by the beam resolution R. In such a case, thefocal depth is expressed by the following formula:fd=A2×R ² ×√{square root over ( )}Vacc/√{square root over ()}(1+0.73×(Ip/B0)×10¹⁴)  (2)Where A2 denotes a constant; Ip denotes a probe current; and BOrepresents the intensity of an electron gun per 1V. In the highmagnification range, many conditions contributing to image formationinfluence a focal depth as shown in the formula (2). With the fieldemission type electron source, which has a very high intensity BO, theitem Ip/BO in the formula (2) is very small, and, therefore, the focaldepth in the high magnification range can be practically expressed as:fd=A2×R ² ×√{square root over ( )}Vacc  (3)Incidentally, the beam resolution R in the formulas (1) through (3) canbe expressed by the following relationship:R=0.61λ/α=0.75/(α×√{square root over ( )}Vacc)  (4)Therefore, the beam resolution R in the formulas (1) through (3) can bereplaced with the second item or third item of the formula (4). Here, λdenotes an electron wavelength, and a denotes the convergent angle (halfangle) of a primary beam.

An image of a focal depth larger than those indicated by the formulas(1) through (3) can be obtained by performing appropriate imagecomposition by use of a plurality of images each having a differentfocus. At that time, the maximum extension effect of a focal depth witha minimum of images can be obtained by setting the focus shift amount toa same value or a little smaller value than those indicated by theformulas (1) through (3).

A focus shift amount determination means calculates an optimal focusshift amount based on image forming conditions such as an accelerationvoltage, the intensity of an electron source, a probe current, thenumber of pixels, a magnification, a beam resolution, using the formulas(1) through (3) The focus shift amount determination means can storethese calculation results in a table beforehand, and select onecorresponding to a given image forming condition from the table. Theminimum number of captured images necessary for optimal composition isalso determined by employing a means for setting the upper and lowerlimits of a desired focal range (range of focal depth) or a means fordirectly entering values specifying a range of focal depth, usingresults from the formulas (1) through (3). This embodiment furtherincludes a focus control means for changing a focus each time an imageis captured; an image sequential-capturing means for sequentiallycapturing a series of images; and an image storage means for storing aseries of images.

The focus control means can be realized in various control modes such ascontrolling a focus using a current focus condition as a center point,or controlling a focus using a current focus as an end point of thecontrol range, or controlling a focus within a predetermined focalrange. This embodiment further includes a means for calculating thefocal depth of an image composed from a plurality of captured images,using the amount of a change in focus between images, the number ofimages, and a focal depth of the electrooptic system, and a displaymeans for displaying the acquired value so that an observer can easilycheck the focal depth of an acquired image.

When it is necessary to perform image composition (focus composition)for extending a focal depth independently from the functions of ascanning electron microscope, a plurality of images stored in the imagestorage means can be provided to another image composing means tocompose an image having an increased focal depth.

This embodiment further includes an image constructing means for furtherconstructing a composite image from a series of images in order toperform a series of processes, such as capturing of a series of images,and indication and storage of a composite image having an increasedfocal depth, together at once.

Since each of a plurality of images used to extend a focal depth hasfocal position information on the object lens, an image position in thecomposite image can be specified to determine the focal positioninformation on the object lens corresponding to the image position. Toutilize this information, this embodiment further includes an imageposition specifying means for specifying two arbitrary points in acomposite image; a focal position extracting means for extracting theobject lens focal positions corresponding to the two specified points;and a means for calculating the height difference between the twospecified points using the extracted focal position information, anddisplaying the results.

This embodiment will be described below with reference to theaccompanying drawings.

FIG. 38 is a schematic diagram showing a configuration of a scanningelectron microscope according to this embodiment. A voltage is appliedbetween a cathode 6001 and a first anode 6002 by a high voltage controlsource 6020 controlled by a microprocessor (CPU) 6040 so that a primaryelectron beam 6004 is extracted from the cathode 6001 at a givenemission current. Since an acceleration voltage is applied between thecathode 6001 and a second anode 6003 by the high voltage control source6020 controlled by the CPU 6040, the primary electron beam 6004 emittedfrom the cathode 6001 is accelerated to proceed to the latter stage of alens system. The primary electron beam 6004 is converged by a converginglens 6005 controlled by a lens control source 6021, and after theunnecessary regions of the primary electron beam are removed by adiaphragm 6008, the primary electron beam is converged into a small spoton a sample 6010 by a converging lens 6006 and an object lens 6007controlled by a lens control source 6022 and an object lens controlsource 6023, respectively. The object lens 6007 can take various formssuch as an in-lens system, an out-lens system, and a snorkel system(semi-in-lens system).

The primary electron beam 6004 is scanned on the sample 6010 by ascanning coil 6009 two-dimensionally. Secondary signals (sample signals)6012 a and 6012 b of secondary electrons, reflection electrons, etc.generated or reflected from the sample 6010 irradiated with the primaryelectron beam proceed to the upper part of the object lens 6007, and areseparated according to their energy by a crossed electromagnetic fieldgenerator 6011 for secondary signal separation, to proceed towardsecondary signal detectors 6013 a and 6013 b. Then, these secondarysignals 6012 a and 6012 b are detected by the secondary signal detectors6013 a and 6013 b. Signals from the secondary signal detectors 6013 aand 6013 b are passed through signal amplifiers 6014 a and 6014 b,respectively, and stored in an image memory 6025 as image signals. Theimage information stored in the image memory 6025 is displayed in animage display device 6026 as necessary. A signal to the scanning coil6009 is controlled by a scanning coil control source 6024 according tothe observation magnification. A plurality of images each having adifferent focus are captured one after another while their focus controlconditions are calculated by the CPU 6040, and the images are stored inan image memory 6032. The image data stored in the image memory 6032 canbe output from the SEM to the outside. Furthermore, images in the imagememory 6032 are subjected to image processing in the CPU 6040, and areused to compose an image having an increased focal depth, which isstored in an image memory 6025 and displayed in the image display device6026. The composite image can also be stored in the image memory 6032,and output from the SEM to the outside. It should be noted that althoughthe image processing can be performed by a program stored in the CPU6040, it may be performed at high speed using dedicated hardware. Sincethe dedicated hardware can provide high speed image processing, it ispossible to capture a series of images each having a different focuswhile applying image processing to the captured images in parallel inorder to compose an image having an increased focal depth.

In an SEM, in addition to the CPU controlling hardware, another computermay be incorporated to provide data processing and a man-machineinterface. In this case, a series of images are temporarily stored inthe image memory 6032 incorporated in the CPU 6040, and then their datais transferred to a computer 6042 for data processing. The image datatransferred to the computer 6042 is processed by a program on thecomputer 6042 to compose an image having an increased focal depth. Thiscomposite image is displayed in a monitor 6043 connected to the computer6042.

In the configuration shown in FIG. 39, electrodes 6016 symmetrical aboutthe axis are provided in the object lens section. These electrodes arearranged such that, their potential distribution is at least partiallyoverlapped with the magnetic field of the object lens, and the focalposition of the primary electrons is changed by controlling the voltageof the electrodes using a control source 6017. Furthermore, as anotherfocal position control means, a coil other than that for the object lensmay be provided in proximity to the object lens 6007 to change the focalposition of the primary electrons by changing the exciting current forthe coil. Further, in addition to the electrodes 6016 which aresymmetrical about the axis and accelerate the primary electrons, acontrol source 6019 for applying a voltage to a sample may be providedin the object lens section, and the voltage applied to the sample may becontrolled by the control source 6019 to change the focal position ofthe primary electrons.

FIG. 40 shows a control flow for capturing a series of images with thenumber of images to be captured specified. An operator can input thenumber of images to be captured by directly entering the number ofimages in the number-of-images setting screen, or selecting the numberof images from a predetermined selection range. The control CPUcalculates the focal depth of a beam based on the current image formingconditions (magnification, acceleration voltage, beam resolution, numberof pixels, etc.), and determines an optimal focus change amount usingthe calculation results and the number of images specified. In anothersetting screen, the operator can further specify the under-focusdirection, the over-focus direction, or both directions with the currentvalue at the center for focus control when capturing images. The controlCPU sequentially captures the specified number of images whileperforming focus control according to the specified control conditions.These images are stored in an image memory, and used for the laterprocesses (image transfer, image composition, etc.)

FIG. 41 shows a control flow for capturing a series of images with anecessary focal depth value specified. In this case, an operatordirectly specifies a necessary focal depth value. The control CPUdetermines the number of images and a focus shift amount necessary tosatisfy the specified focal depth using the focal depth of a beam, andcaptures and stores a series of images using the same procedure as thatshown in FIG. 40.

FIG. 42 shows a control flow for capturing a series of images with theirfocal range specified. In this case, an operator specifies the upper andlower limits of a desired focal range in an observed image. First, aportion of a sample to be set to the lower limit of the focal range isset in focus, and this focal condition is registered in the CPU as thefirst focal condition. Next, a portion of the sample to be set to theupper limit is set in focus, and this focal condition is registered inthe CPU as the second focal condition. The control CPU calculates thefocus control range based on these registered conditions, and determinesthe suitable number of images and a suitable focus shift amount from thefocal depth value of a beam.

By composing an image using a plurality of images captured in variousforms according to this embodiment, an operator can construct an imagehaving a desired focal depth. Furthermore, in this case, it is possibleto restrict the amount of beam irradiation (proportional to the numberof images) on a sample necessary to capture images, to a theoreticalminimum value, minimizing the beam damage and shortening imageacquisition and processing times.

Incidentally, description has so far been made with reference to chargedparticle beams. Techniques for optically captured images, on the otherhand, are disclosed in Japanese Laid-Open Patent Publications Nos.5-333271 (1993), 9-26312 (1997), 6-113183 (1994), 2000-39566 (2000),5-227460 (1993), 10-290389 (1998), 11-261797 (1999), 11-283035 (1999),and 11-287618 (1999).

1. A method for measuring a dimension of a pattern on a sample,comprising steps of: scanning a charged particle beam on the pattern;detecting charged particles being caused by the charged particle beamscanning on the sample; selecting an area for part of the sample on asignal which is formed by the detected charged particles; and measuringa dimension of the pattern based on the signal except the selected areaon the signal.
 2. The method for measuring the dimension of the patternas claimed in claim 1, wherein the selected area on the signal ischanged into another signal.
 3. The method for measuring the dimensionof the pattern as claimed in claim 1, wherein in the selecting step, anarea having pre-determined focus evaluation value is selected.
 4. Themethod for measuring the dimension of the pattern as claimed in claim 1,wherein in the selecting step, a pre-determined area is changed intoanother image.
 5. A pattern measurement apparatus using a chargedparticle beam comprising: a charged particle source for emitting thecharged particle beam; a scan deflector for scanning the chargedparticle beam emitted from the charged particle source on a sample; acharged particle detector for detecting charged particles emitted formthe sample; and a data processing system configured to perform themachine-implemented steps of: selecting an area for part of the sampleon a signal which is formed by the detected charged particles; andmeasuring a dimension of the pattern based on the signal except theselected area on the signal.
 6. The pattern measurement apparatus asclaimed in claim 5, wherein the data processing system is furtherconfigured to change the selected area on the signal into anothersignal.
 7. The pattern measurement apparatus as claimed in claim 5,wherein the data processing system selects an area having pre-determinedfocus evaluation value in the selecting step.
 8. The pattern measurementapparatus as claimed in claim 5, wherein the data processing systemchanges a pre-determined area into another area.